You might want to examine the way that the narrator, Mama, introduces Maggie to us, and in particular the way that she is presented as being a profoundly shy and timid young woman who is unable to assert herself against anybody and has a very low self-esteem. Consider the way that Mama introduces her daughter:
Have you ever seen a lame animal, perhaps a dog run over by some careless person rich enough to own a car, sidle up to someone who is ignorant enough to be kind to him? That is the way my Maggie walks. She has been like this, chin on chest, eyes on ground, feet in shuffle, ever since the fire that burned the other house to the ground.
We can therefore see how the scars that Maggie bears are at least partly responsible for her withdrawn nature and her shyness and lack of confidence. Even when she asks her mother how she looks, she still hides behind the door, only showing enough so her mother knows that she is there. The physical disfigurement seems to operate as a symbol of her psychological disfigurement.